1. Field of the Invention
The present general inventive concept relates to an illumination unit emitting light and a projection type image display apparatus employing the same, and more particularly, to an illumination unit with an improved prism to synthesize light emitted from a compact light source and a projection type image display apparatus employing the illumination unit.
2. Description of the Related Art
In general, illumination units include a light source emitting light in one direction and an illumination optical system projecting the light emitted from the light source. The illumination units are widely employed in projection type image display apparatuses that produce an image using an image-forming device (e.g., a liquid crystal display (LCD) device or a digital micromirror device (DMD)) that cannot emit light.
In recent years, illumination units and projection type image display apparatuses using a small light-emitting device, such as a light-emitting diode (LED) or a laser diode, as a light source have been developed.
Since the small light-emitting devices can emit beams of red, blue, and green wavelengths, single-panel projection type color image display apparatuses employing the small light-emitting devices do not need an additional color wheel for obtaining a color image. However, to emit various color beams, a plurality of small light-emitting devices and a structure for synthesizing the various color beams are required.
FIG. 1 illustrates a conventional illumination unit. Referring to FIG. 1, the conventional illumination unit includes first, second, and third LED light sources 11, 12, and 13 disposed at different positions and respectively emitting beams of red, blue, and green wavelengths, and a trichroic prism 20 propagating the beams emitted from the first, second, and third LED light sources 11, 12, and 13 along the same path with respect to each other.
The trichroic prism 20 includes first, second, and third prisms P1, P2, and P3, a first color filter 21 disposed between the first prism P1 and the third prism P3, and a second color filter 25 disposed between the second prism P2 and the third prism P3. The first and second color filters 21 and 25 selectively transmit or reflect incident light according to a wavelength of the incident light. For example, the first color filter 21 reflects a first beam R of a red wavelength and transmits a second beam G of a green wavelength and a third beam B of a blue wavelength. The second color filter 25 reflects the third beam B and transmits the first and second beams R and G.
Accordingly, the first beam R incident on an exit surface 20a of the first prism P1 from the first LED light source 11 is totally reflected based on the principle of critical angle total reflection, and reaches the first color filter 21. The first beam R is reflected by the first color filter 21 and is then transmitted through the exit surface 20a of the first prism P1. The second beam G is sequentially transmitted through the second and first color filters 25 and 21, and propagates along the same path as the first beam R. The third beam B is totally reflected based on the principle of critical angle total reflection on a surface 20b of the third prism P3 facing the first prism P1 to be directed toward the second color filter 25, reflected by the second color filter 25, transmitted through the first and third prisms P1 and P3, and then propagates along the same path as the first and second beams R and G. Hence, the first, second, and third beams R, G, and B respectively emitted from the first, second, and third LED light sources 11, 12, and 13, which are disposed at different positions, are synthesized to propagate along the same path with respect to each other.
For the purpose of totally reflecting the third beam B on the surface 20b of the third prism P3, the first prism P1 and the third prism P3 are spaced a predetermined distance from each other to form an air gap Gair therebetween. That is, to cause critical angle total reflection, there must exist a refractive index difference between the third prism P3 and a surrounding medium, as well as an angle between the surface 20b of the third prism P3 and the third beam B.
The second prism P2 is required to make a length of an optical path from the second LED light source 12 to the exit surface of the first prism P1 equal to a length of an optical path from the first and third LED light sources 11 and 13 to the exit surface 20a of the first prism P1. An illumination unit employing a light source with a narrow angle distribution causes little light loss even though optical paths of a plurality of beams are different from one another.
Nevertheless, the use of the second prism P2 increases the overall size of the trichroic prism 20, thereby making miniaturization difficult and increasing light loss and manufacturing costs.